NON-THERMIONIC DIODES 



pass about 3 mA, though a few types pass 7 or 8. With 10 V across them in 

 the backward direction the leakage current may be anything from a few 

 microamperes to a milliamp. As a general rule, germanium diodes with a 

 high P.I.V. have low reverse currents. There is also a slight tendency for 

 diodes with very low forward resistances to have a poor P.I.V., but the 

 reverse is not so; the STC 2X/106, for example, has a good P.I.V. — 70 V — 

 and passes a forward current of 7 mA at 1 V. On the whole the high P.I.V. 

 diode is the better component. 



Selenium type 



These are among the smallest electronic components made, and are rather 

 lower-power devices. The STC Ml and M3 diodes, for example, have a 

 P.I.V. of 68 V but can pass mean forward currents of only 250 microamps 

 and 1 milliamp respectively. The backward resistances are enormous — 1,000 

 megohms for the Ml and 45 megohms for the M3 — but the forward resis- 

 tances are also rather high, 10 k and 1-2 k. Ml can be used at frequencies 

 up to 5 M/c, but M3 only up to 100 kc/s. 



Copper-oxide type 



These have a P.I.V. of only 6 V per junction, and so have generally to be 

 assembled into stacks. Copper-oxide signal diode stacks are made for a 

 P.I.V. between 6 and 90 and for currents ranging from 100 jjlK to 10 mA. 

 They occupy a position intermediate between the germanium diode and tiny 

 selenium Ml and M3 so far as forward current rating is concerned. The 

 reverse current characteristic of copper-oxide rectifiers is rather worse than a 

 good germanium diode, but the forward current reaches its rated maximum 

 when the forward voltage is only 0-7. This, coupled with their stability of 

 characteristics, makes them suitable for instrument (i.e. meter) applications. 



Silicon junction type 



These are not of the cats-whisker variety and are not suitable for use at 

 frequencies above 100 kc/s or so. Their most important characteristic is a 

 remarkable front-to-back ratio. Their forward resistance is only about 10 

 ohms, whilst the backward resistance is commonly hundreds of megohms. 

 P.I.V. 's lie in the range 60-180 V and maximum mean forward currents are 

 100 mA. The excellent performance is combined with ability to operate — 

 with reduced ratings — in ambient temperatures up to 150°C. 



A group of comparable signal diodes is shown in Plate 23.4. No. 1 is 

 germanium and whisker, P.I.V. 90, /max 50 mA. No. 2 is copper-copper- 

 oxide, P.I.V. 72, /max 250 fxA. No. 3 is selenium iron, P.I.V. 68, /max 250 ^A. 

 No. 4 is a silicon junction unit, P.I.V. 120 V, /max 100 mA. 



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